It is known that so-called knocking can take place in a gasoline engine under particular operating circumstances. By such knocking is understood the generation of shockwaves of the fuel-air mixture which are noticeable as audio frequency vibrations of the motor and sometimes otherwise too. Since a heavy thermal loading of the internal cylinder walls and of the piston takes place as a rule when the engine knocks, with the possibility of wearing off material from these surfaces, efforts are made for basically avoiding knocking of the engine, since prolonged appearances of that phenomenon can lead to damage or destruction of the engine.
Since efforts are also made to utilize as far as possible the available working range of the engine (with regard to an optimum development of torque or of combustion efficiency), there is a need for recognition of knocking of the engine as soon and as reliably as possible. Along with the problem of making available suitable sensors for detecting engine knock, there is also the problem of measurement technology to read out the knock signal reliably and free of disturbance from the oscillations of the engine detected by the sensor, in order to be able to control the engine in a manner corresponding to the presence of a knock recognition signal that is either positive ("knock yes") or negative ("knock no").
A method and apparatus for recognizing engine knock is shown in German Published Patent Application DE-OS No. 31 37 016, which corresponds to U.S. Pat. No. 4,478,068 and in which a useful signal relates to engine knock is compared with a reference signal. The reference signal in this case is formed after the fashion of an output sequence of a digital filter and provides, more particularly, a recursive function made up repetitively from the latest useful signal value and the last previous value of reference signal. For a knock recognized as validly shown, an output signal is produced when the useful signal exceeds the reference signal in a prescribed way. A problem in such a process or such an apparatus is that in the case of a failure of the detection of the useful signal no error recognition is possible, which can lead to damage to the engine from the knock control system subsequently providing more or less continuous spark advance.
A knock detector is also disclosed in U.S. Pat. No. 4,012,942 in which the measured knock signal is compared with a reference signal which is generated by a function generator in a manner dependent on the engine speed. This method has the disadvantage that the background noise of the engine which in fact is present is not taken into consideration. In this system, notably, no account is taken of the particular mode of the engine's operation, and no account is taken of the engine's various adjustments or the aging condition of the engine.